Subject: Re: DEBUG_PAGEALLOC on PPC not working (kernels 2.6-25, 3.0-34)

From: Scott Wood <[email-masked]>
Date: Wed, 19 Jun 2013 16:00:46 -0500

On 06/19/2013 08:09:44 AM, saikia.partha wrote:
> Hi,
>=20
> We have two Freescale PowerPC boards on which we're trying to enable
> DEBUG_PAGEALLOC with the hope that we'll see an exception whenever =20
> some
> code tries to modify a page that's been already freed. To test it, we =20
> wrote
> this sample code -
>=20
> =3D=3D=3D
> #define BUF_SIZE    4096
> void
> pagealloc_test(void)
> {
>     char *buf =3D kmalloc(BUF_SIZE, GFP_KERNEL);
>=20
>     if (!buf) {
>         printk("%s[%d] - alloc failed!\n", __func__, __LINE__);
>         return;
>     }
>     printk("%s[%d] - alloc'd\n", __func__, __LINE__);
>     memset(&buf[0], 0, BUF_SIZE);
>     printk("%s[%d] - memset'd\n", __func__, __LINE__);
>     kfree(buf);
>     printk("%s[%d] - free'd\n", __func__, __LINE__);
>     memset(&buf[0], 1, BUF_SIZE);
>     printk("%s[%d] - memset'd after free!\n", __func__, __LINE__);
> }
> =3D=3D=3D
>=20
> Here, the last memset() should generate an exception if PAGEALLOC code
> correctly unmapped the page during kfree(). However, kernel is happily
> running after the memset post-free. Any clue?
> Also, the 2nd board has Book-E which has a different MMU architecture.
> Here, I saw that the function __change_page_attr() always returns as =20
> the
> condition v_mapped_by_tlbcam() is true. How do we remove the pte =20
> entry from
> the TLB in this case?  Has anyone tried DEBUG_PAGEALLOC on Book-E?

This is not supported on booke -- the tlbcam mapping is required for =20
all lowmem.

> Please find the board details below -
> Board 1: CPU - e300c4, kernel 2.6-25

For e300, I think I had it working at one point a few years ago (see =20
commit bde6c6e16aa489ea76c762fb7ffb0abb48660dd8).

The reason we can do it on e300 and not on booke is because e300 takes =20
exceptions in real mode.  On e500 the MMU is always enabled, so we need =20
bolted TLB1 entries that cover at least all exception code (up to the =20
point where a TLB miss could safely be taken) and all page tables (in =20
practice, we just bolt all lowmem) and other data that can be =20
referenced from said exception code.  There are not enough TLB1 entries =20
to do this on a fine-grained basis.

-Scott=